Unlike a marathon, it was the first few miles of Denis Asselin's walk to Boston that proved the most grueling.

On Tuesday he visited Bonner Park in Thornbury, the place his 24-year-old son Nathaniel decided to end his life last April with a bullet to the head.

Wednesday brought more tears during a stop at The Birth Center at Bryn Mawr Hospital, where Nathaniel entered the world.

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In between, Denis met with the psychiatrist who treated his son for Body Dysmorphic Disorder (BDD), a condition that causes people to obsess about minor or perceived flaws in their appearance.

"Nathaniel had a heightened ability to see things that other people didn't see," said Denis, 63, a retired teacher from Thornbury Township, Chester County. "He was an incredibly handsome young man who didn't see himself that way."

Nathaniel's mental illness surfaced at age 11 when he started to run obsessively.

"He began to lose a lot of weight and we had to hospitalize him," Denis said. "It was tough to figure out exactly what the problem was because he was just entering pre-adolescence. At first, we thought it might be anorexia."

Once properly diagnosed, Nathaniel tried a range of treatments, cognitive therapy and a combination of medications to help him combat the disease.

"He was an amazing person -- kind, funny, caring," Denis said. "But he had a miswiring in the brain that caused the same messages and misperceptions to keep going around in a closed loop. He was in extreme agony all the time and it finally became more than he could bear."

Denis said the family tried everything to alleviate Nathaniel's suffering.

"As a parent, I told him how I wished I could carry it for him, even for a little while, to make the pain go away," he said. "His response was, 'I would never want you to have this for a single day.'"

After taking a trip to Spain last summer with his wife Judy and daughter Carrie that included walks along an ancient Christian pilgrimage route known as El Camino de Santiago, Denis returned home with "an irresistible urge" to keep walking.

"I knew there was more walk in me," he said. "I began to think, 'If I went out my front door on a pilgrimage, what would that route look like?'"

Eventually, he put together a "Camino de Nathaniel," a 500-mile journey that would take him to many of the happy and not-so-happy places and markers of his son's life.

"I wanted to do it as a grounding experience for me but, more importantly, to tell Nathaniel's story and raise awareness of brain disorders which, for me, are kind of the last frontier of medicine," he said. "It's an area so complex and beyond our understanding that people tend to just shy away from it.

"This walk is about making the invisible visible and hopefully making it easier for other people to share their stories."

Denis is also soliciting donations for the International Obsessive Compulsive Disorder (OCD) Foundation in Boston.

Nathaniel Asselin attended the Shipley School and Westtown School before his condition made mainstream schooling impossible. He graduated in 2005 from the Upattinas School in Glenmoore, Chester County and later took classes at Delaware County Community College.

He worked as the assistant director of the middle school afterschool program at Westtown, where his mother still teaches. (Denis taught at Shipley and Westtown for 36 years.)

Nathaniel also coached the middle school cross-country program and volunteered hundreds of hours at the Good Fellowship Ambulance Club in West Chester.

"He was a pied piper with the kids and was always looking to help people," Denis said. "He was very thoughtful and caring person."

According to his father, Nathaniel's anguish was compounded by memories of pain-free living. "Part of what made it so painful is that he could remember what it was like to live without it," Denis said.

Over the years, Nathaniel's circle of activity began to shrink to the point where it became difficult to go out in public.

"He believed that people were seeing him the way he saw himself and that was terrifying to him," said Denis, who spent part of his Thursday discussing his journey with Shipley students.

As it stands now, Denis expects to head north toward Allentown and then turn east, walking through New Jersey and out to the tip of Long Island, where he will hop a ferry to New London, Conn., and press on to Boston.

Along with his iPhone, he packed two pairs of running shoes, some quick-dry clothing, a fleece jacket, rain coat and a few other essentials.

In addition to donations to the OCD Foundation, Asselin is accepting whatever kindness comes his way from strangers he meets on the road.

"That might be a meal or a discount for a hotel -- anything that helps keep the cost down," he said.

His itinerary, which presently has him arriving in Boston June 7, is far from fixed.

"There is only so much organization you can do," he said. "You never know what the path will hold."